Well there's very good reasons why OTR can fit better, well sort of. First off when shopping you put something on and generally see if it works or not, if it doesn't you discard it until you find a cut that works, with bespoke it's got to be right in one shot.
Secondly OTR suits are made to fit a lot of people "ok" so those armholes are big for example. now if you put on a bespoke suit made for you and the armholes are a touch too close, you will feel uncomfortable and that will draw attention to the area.
Finally when getting something made for you, you will probably start to scrutinize the garment a whole lot more. If your looking for faults you will probably find them, because nothing's perfect. Have a big lunch? That immaculately fitted vest next week may be a tad tight. Had some coffee today? Your posture is different from when you come in and your tired. These are slight things that you may pick up on if your looking hard enough.
One of the joys of bespoke, (and I am my best customer) is the process. If your like oh I hate coming in for fittings and can't be bothered waiting a long time for something, then your missing a part of/not appreciating the experience, and those fittings and that time is actually what you have paid for in many respects.
Finally of course there is the prestige of having something made for you and you alone, imagine getting anything else made for you personally like say your car, your watch, your home, how good would that be?